1. Field of the Invention
This invention relates to the manufacture of semiconductor devices and more particularly relates to a process for the deposition and diffusion of an impurity into a semiconductor substrate.
2. Art Background
One of the most important steps in a process for fabricating semiconductors is the diffusion of impurities into a semiconductor substrate. One of the usual first steps of this process is the deposition of a controlled quantity of impurities into a surface film of the semiconductor. The impurities are then distributed and driven into the semiconductor as a result of high temperature solid state diffusion. While the second step has in the past been reasonably well defined and controlled, the first step has been subject to more variation.
One of the most widely used deposition methods is known as hot wall open tube furnace deposition. In this method, semiconductor substrates are held on carriers and placed into a furnace which is purged by a controlled gas composition. Dopant impurities in gas or vapor form are mixed into the purging gas in a predetermined concentration. A generally accepted explanation of the process is that active dopant molecules rapidly react with the semiconductor surface and as a result a stagnant dopant-free gas film develops above the semiconductor surface. Deposition of dopant molecules into the surface occurs as a result of diffusion through the stagnant gas film. The stagnant dopant free gas film impedes the diffusion process, and as a result, doping speed is defined by a complicated function of temperature, composition and fluid properties of the gas, and surface properties of the semiconductor. Simultaneous control over all of these parameters is difficult. Consequently, control over the entire doping process is rather poor. Also, because of the generally very high concentration of dopant atoms on the surface of the semiconductor body, lattice distortions tend to occur which detract from the desired electronic characteristics of the resultant semiconductor body.
As an alternative to the aforementioned diffusion process, a second deposition technique known as the ion implantation method is commonly used. In this method dopant atoms are ionized and are then accelerated in the form of a high energy ion beam which is directed onto the semiconductor surface. The ions penetrate into the surface to a depth which is proportional to the energy of the beam. Variation of the ion beam intensity provides a means for precisely controlling impurity depth distribution and doping levels in the semiconductor.
The ion implantation method has an advantage over the above mentioned diffusion process in that it is an externally controlled non-equilibrium process, whereas the diffusion process is characterized by unchangeable physical conditions such as the diffusion constant of the impurity in the semiconductor material. Unfortunately, considerable damage is done to the semiconductor crystal structure by the high energy ion penetration, and the ion stream has been known to drag impurities from remnant gas and from the semiconductor surface into the semiconductor.
A method known as sealed tube doping was in frequent use during the early days of semiconductor device fabrication. According to this method, the semiconductor substrate and a solid-state dopant are sealed in an evacuated tube. The tube is then heated to a predetermined temperature whereby the solid state dopant evaporates and diffuses into the semiconductor substrate. Control of the doping level by variation of temperature and dopant quantities is possible but this method is not applicable for mass production.
All prior art methods for diffusing active impurities into a semiconductor substrate suffer from an inability to provide optimal dopant concentrations and distributions and to eliminate unwanted impurity inclusion in the semiconductor substrate. The prior art methods also suffer from an inability to provide precise and accurate control of simultaneous deposition of two or more dopants. The simultaneous deposition of two or more dopants is for example necessary in the production of certain devices requiring semiconductor layers with very high dopant concentrations and high carrier lifetimes. Bipolar transistors with high efficiency emitters, and solar cells are but two examples of such devices.